Size the bathroom vent to the fan.
A bathroom fan is sold on its airflow, and the vent face is where that air leaves the room. Size the face to the fan, and the air leaves within published design limits. Start with where the fan sits.
- If your fan is inline or remote, in the attic or a chase: pick the face for the fan. On 4-inch duct, the face can be round (4, 5 or 6 inches) or square (4x4 or 6x6). In a larger bathroom, one fan can serve two faces.
- If your fan is mounted in the ceiling with its own cover: HiFi makes a custom flush face sized to the fan's cover, round or square.
If your fan is inline or remote
An inline fan sits in the duct between the bathroom and the outside; a remote fan sits farther along it. The room sees only the vent, so the face is a choice: pick it for the fan.
Pick the face for the fan
Each HiFi face, round or square, is sized for a fan's airflow in CFM (cubic feet per minute). For an inline or remote fan, every face here connects directly to 4-inch duct:
| Your bath fan | The face |
|---|---|
| About 20 CFM, continuous | 4-inch round or 4x4 square |
| Up to about 35 CFM | 5-inch round or 6x6 square |
| Standard 50 CFM | 6-inch round or 6x6 square |
| Mounted in the ceiling, with its own cover | A custom face sized to the fan's cover, round or square |
20 CFM is the code minimum for a bath fan that runs all the time; 50 CFM, for one switched on when the room is in use. The 5-inch round also comes for 5-inch duct, the 6-inch round for 5- or 6-inch duct, and the 6x6 square for 6-inch duct.
Every size is your choice of a removable grille, which lifts out for cleaning, or a fixed face, in round, square or any pattern, made to fit. See the 4-inch round as a fixed face or a magnetic removable grille, the 4x4 square as a fixed face or a removable grille, the 5-inch round, the 6-inch round and the 6x6 square.
How each face is sized
A bath exhaust vent takes air in, as a return grille does. Published design limits for those openings are about 300 fpm (feet per minute) averaged across the whole grille face and about 600 fpm through its open area. Each HiFi face, round or square, is sized where both limits hold; the round faces are designed for about 52 percent open area. On the 5- and 6-inch round faces and the 6x6 square, the 4-inch duct connection opens to the full 4-inch inside diameter, so a 50 CFM fan's air passes through it under 600 fpm as well.
By HiFi's calculation, at each face's sized airflow:
- 4-inch round, 20 CFM: about 280 fpm across the face; 530 through the open slots.
- 4x4 square, 20 CFM: about 200 to 220 fpm across the face; under 520 through the openings.
- 5-inch round, 35 CFM: just under 300 fpm across the face; about 570 through the slots.
- 6x6 square, 35 CFM: about 160 fpm across the face; under 380 through the slots; under 500 in its 4-inch duct connection.
- 6-inch round, 50 CFM: about 290 fpm across the face; 550 through the slots; 570 in its 4-inch duct connection.
- 6x6 square, 50 CFM: about 230 fpm across the face; under 540 through the slots; 570 in its 4-inch duct connection.
The speed matters because the resistance at an opening rises with the square of the air's speed (ASHRAE Handbook, Fundamentals), and a fan moves less air as the resistance against it rises. What a fan delivers depends on everything in its path: vent, duct, elbows and wall cap. A face sized to the fan keeps the vent's share of that low.
Sources: calculated by HiFi from its proprietary design data, 2026-10-07: airflow divided by face area, open area and duct connection area, with the round faces designed for about 52 percent open area; not a test result. 300 fpm: U.S. Department of Energy, Building America Solution Center, "Compact Air Distribution" (face velocity, return and transfer grilles). 600 fpm: ASHRAE Handbook, HVAC Applications 2023, chapter 49 (return openings, free-opening velocity, bathrooms). 20 and 50 CFM: 2021 IRC, Table M1505.4.4; ASHRAE Standard 62.2.
A bigger face on the same duct
HiFi's patent-pending design and manufacturing process lets the same flush face come round, in 4-, 5- and 6-inch sizes, or square, in 4x4 and 6x6, each made to connect directly to 4-inch duct. So a 50 CFM fan keeps the 4-inch duct it already uses and gets a 6-inch round or 6x6 square face, sized for its air.
The flange is plastered into the ceiling or wall and painted with it, so the room sees the face, not an applied rim (installation notes).
Bigger fans
Above about 50 CFM, the air inside a 4-inch duct itself passes 600 fpm, and the code allows a bigger fan less 4-inch duct (2021 IRC, Table M1504.2), so larger fans often run on 5- or 6-inch duct. For a bigger fan, send us its model number and the duct size, and we'll size the face to it.
Size with the fan's rated airflow at static pressure, the rating the code's duct table uses, not a free-air figure, which overstates what the fan moves once installed.
Two faces on one fan
In a larger bathroom, one inline or remote fan can serve two smaller faces, for example one near the shower and one near the toilet. A Y splitter (a wye fitting) in the duct sends about half the fan's air to each face, so size each face for half the fan's airflow:
- 50 CFM fan: two 5-inch round or 6x6 square faces, about 25 CFM each.
- 80 CFM fan: two 6-inch round or 6x6 square faces, about 40 CFM each.
- 100 CFM fan: two 6-inch round or 6x6 square faces, about 50 CFM each.
Each face connects directly to its own 4-inch branch. Keep the two branches about the same length, with the same number of bends, so the air divides evenly, and size the duct from the splitter to the fan for the fan's full airflow (see Bigger fans, above). Split two ways, a fan up to 100 CFM keeps the air in each 4-inch branch under 600 fpm: about 460 fpm at 40 CFM per branch and 570 at 50.
If your fan is mounted in the ceiling
A ceiling exhaust fan keeps its motor in a housing above the ceiling, behind its own cover. HiFi makes a custom flush face to take the place of that cover, sized to it: round or square, linear or any pattern. For example, a common ceiling fan has a cover about 13 inches square; HiFi sizes the face to that cover. Removable or fixed is your choice: a removable grille lifts out for cleaning and for the fan's switch, and a fixed face stays in place. Either way, its plaster-in frame is mudded in, finished into the drywall and painted with the ceiling.
Tell us your fan's model number, whether it's new construction or a finished ceiling, the color and when you need it, and we'll match the face to the fan. Send us the details, or see custom options.
Placement
Near or over a shower or tub, place the vent where the fan maker's instructions and the local code allow an exhaust inlet. HiFi's vents are made of PETG, rated by HiFi for 140 degrees F (60 C) continuous. Keep the duct short, with few bends: a fan rated 50 CFM is allowed 56 feet of 4-inch flexible duct or 114 feet of smooth, less 15 feet per elbow (2021 IRC, Table M1504.2). Run it outdoors, never into an attic, soffit or crawl space (2021 IRC, section M1501.1), and leave access to the fan.
Small rooms
Toilet and powder rooms have the same code minimum as bathrooms, 50 CFM intermittent or 20 continuous (2021 IRC, Table M1505.4.4), so the same sizing applies.
Cleaning
Removable or fixed is your choice. A removable grille lifts out for cleaning and, over a ceiling fan, for the fan's switch; don't seal that seam or the openings. A fixed face stays in place for cleaning; keep its openings clear.
Questions
What is an inline bathroom fan? A fan mounted in the duct between the bathroom and the outside, so the bathroom sees only the vent.
What's the difference between an inline fan and a ceiling exhaust fan? Where the motor sits: in a ceiling housing behind its own cover, or in the duct, with the vent chosen separately. HiFi makes the vent for either. More on choosing between them: Planning a bathroom with a remote fan.
Can HiFi make a grille for a ceiling-mounted bath fan? Yes: a custom flush face sized to the fan's cover, round or square, linear or any pattern, removable or fixed as you choose, painted with the ceiling. Send your fan's model number and we'll match it.
Should the grille be removable or fixed? Your choice. A removable grille lifts out for cleaning and for a ceiling fan's switch; a fixed face stays in place. Either one comes round, square or in any pattern, made to fit.
What size vent does a 4-inch inline fan need? Size it to the fan's airflow: a 4-inch round or 4x4 square face for a continuous fan around 20 CFM, a 5-inch round or 6x6 square up to about 35 CFM, and a 6-inch round or 6x6 square for a standard 50 CFM fan. Each connects directly to 4-inch duct.
Can one bath fan serve two vents? Yes, with an inline or remote fan: a Y splitter sends about half the fan's air to each vent, so each face is sized for half the airflow. A 50 CFM fan, for example, takes two 5-inch round or 6x6 square faces.
Can a trimless vent be used for bathroom exhaust? Yes, with an inline, remote or ceiling-mounted fan, placed where the fan maker's instructions and the local code allow.
Ordering
The bathrooms in the lookbook are concept renderings in plaster and tile. For a whole house, send us your list in any format; we'll match each vent to the catalog, confirm it with you and send one invoice. Orders typically ship within 2-3 weeks.